An electric power steering apparatus that energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque (a steering assist torque), a control apparatus for such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current control value and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
Here, a general configuration of an electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft 2 connected to a steering wheel (a handle) 1 is connected to tie rods 6 of steered wheels through reduction gears 3, universal joints 4A and 4B, and a rack and pinion mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1, and a motor (a brushless DC motor) 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 14, and an ignition signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a steering assist torque command value Tref of an assist command based on a steering torque T detected by the torque sensor 10 and a velocity V detected by a velocity sensor 12, and controls a current I supplied to the motor 20 based on calculated steering assist torque command value Tref. Furthermore, it is also possible to obtain the velocity V from a CAN communication network of the vehicle.
The control unit 30 mainly comprises a CPU, and general functions performed by programs within the CPU (or an MCU or an MPU) are shown in FIG. 2. Furthermore, FIG. 2 shows a case of a vector control system that performs current control by means of a d-axis current command value Id and a q-axis current command value Iq.
In the case of the vector control, a rotation sensor (such as a resolver or a Hall sensor) 21 for detecting a rotational position θ is connected to the motor 20, the rotational position θ from the rotation sensor 21 is inputted into a rotational speed calculating section 22, and a rotational speed ω is calculated. The steering torque T, the velocity V and the rotational speed ω are inputted into a steering assist torque command value calculating section 31, and the steering assist torque command value Tref is calculated. A current command value calculating section 32 calculates the d-axis current command value Id and the q-axis current command value Iq based on the steering assist torque command value Tref, the rotational position θ and the rotational speed ω. The d-axis current command value Id, the q-axis current command value Iq and the rotational position θ are inputted into an each-phase current command value calculating section 33. Each-phase current command values Iaref, Ibref and Icref that are calculated by the each-phase current command value calculating section 33, are inputted into subtracting sections for feedback 34a, 34b and 34c, respectively. Each-phase currents ia, ib and ic of the motor 20 that are detected by current detectors 38a, 38b and 38c, are fed back into the subtracting sections 34a, 34b and 34c. Deviations (Iaref-ia, Ibref-ib and Icref-ic) obtained by the subtracting sections 34a, 34b and 34c, are inputted into a PI control section 35. Voltage command values Varef, Vbref and Vcref that are calculated by the PI control section 35, drive and control the motor 20 via a PWM control section 36 and an inverter 37.
In the above-described electric power steering apparatus, since resistance values of three phases between the brushless DC motor 20 and the control unit 30 (i.e. resistance (R) and inductance (L)) are not identical, and are different respectively, as shown in FIG. 3, there is a problem that maximum amplitude values of currents ia, ib and ic that flow in three phases do not become identical, and torque ripple occurs. That is to say, in the case of FIG. 3, with respect to a peak value of the phase current ib, a gap Pa appears in the phase current ia, and a gap Pc appears in the phase current ic. Furthermore, with respect to variation in each-phase current values of the motor, variation factors of the inverter circuit 37, substrates, wiring resistances and so on within the control unit 30 are conceivable other than variation in characteristic of a stand-alone motor.
From the aspect of improving steering performances, reducing operating noises and vibrations due to such a difference in the peak value of the phase current, is strongly requested.
In order to solve such a problem, for example, in Japanese Patent Application Laid-Open No. 2009-81951 (Patent Document 1), a motor control signal generating section comprises a phase resistance correction calculating section, preliminarily-measured each-phase resistance values Ru, Rv and Rw, and a reference resistance value R are stored in the phase resistance correction calculating section, based on these stored values, correction components εd and εq for canceling out a voltage drop item of a voltage equation that varies depending on a rotational angle of the motor, are calculated, the motor control signal generating section corrects a d-axis voltage command value Vd* and a q-axis voltage command value Vq* that aims to suppress occurrence of the torque ripple due to differences in each-phase resistance values by overlaying these correction components εd and εq on the d-axis voltage command value Vd* and the q-axis voltage command value Vq*.
Furthermore, in Japanese Patent Application Laid-Open No. 2010-130707 (Patent Document 2), a three-phase correction section calculates armature winding resistances Ru, Rv and Rw, or rates of winding resistance Gu, Gv and Gw of three phases based on command currents id* and iq* on the d-axis and the q-axis, and a current value is detected by a current sensor, based on the calculated armature winding resistances Ru, Rv and Rw, or the calculated rates of winding resistance Gu, Gv and Gw, a correction for compensating a deviation from a setup value that is due to variation in winding resistance values of three phases, is performed with respect to each-phase command voltages Vu, Vv and Vw that are obtained by a dq-axes/three-phase converting section, even in the case that each-phase winding resistance values are different due to resistance change caused by variation in manufacturing resistances and an ambient temperature, and so on, a deviation of the command voltage due to that difference is corrected, and the torque ripple is reduced.